The present invention relates to a method for recording and reproducing images of objects made by high energy radiation. It relates especially to a method for manufacturing a cesium halide storage phosphor, an more particularly phosphors and a storage phosphor panel containing said phosphors.
In computer radiography (CR), a photostimulable phosphor is used, which being incorporated in a panel, is exposed to incident pattern-wise modulated X-ray beam and, as a result thereof, temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are can be processed in order to produce a visible image. For this purpose, the phosphor should store as much as possible incident X-ray energy and emit as little as possible stored energy until stimulated by the scanning beam.
The image quality that is produced by computer radiography largely depends on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be. This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimized sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapour deposition, which may be thermal vapour deposition, sputtering, electron beam deposition or other of phosphor material on a substrate.
Use of alkali metal halide phosphors in storage screens or panels is well known in the art of storage phosphor radiology and the high crystal symmetry of these phosphors makes it possible to provide structured screens and binderless screens. In U.S. Pat. No. 5,055,681 e.g. a storage phosphor screen comprising an alkali metal phosphor in a pile-like structure is disclosed.
In U.S. Pat. No. 5,736,069 an alkali metal storage phosphor is disclosed corresponding to the formula:
M1+Xxc2x7aM2+Xxe2x80x22xc2x7bM3+Xxe2x80x33:cZ
wherein
M1+ is at least one member selected from the group consisting of Li, Na, K, Cs and Rb,
M2+ is at least one member selected from the group consisting of Be, Mg, Ca, Sr, Ba, Zn, Cd, Cu, Pb and Ni,
M3+ is at least one member selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Bi, In and Ga,
Z is at least one member selected from the group Ga1+, Ge2+, Sn2+, Sb3+ and As3+,
X, Xxe2x80x2 and Xxe2x80x3 can be the same or different and each represents a halogen atom selected from the group consisting of F, Br, Cl, and I, and 0xe2x89xa6axe2x89xa61, 0xe2x89xa6bxe2x89xa61 and 0 less than cxe2x89xa60.2.
In EP-A-174 875 and EP-B-252 991 (and the corresponding U.S. Pat. No. 5,028,509), among other alkali metal stimulable phosphors a CsBr:Eu phosphor is disclosed, wherein Eu is incorporated in the CsBr by firing CsBr with europium oxide.
In U.S. Provisional Patent Application No. 60/142,276, filed Jul. 2, 1999 and U.S. Provisional Patent Application No. 60/159,004, filed Oct. 8, 1999, a novel CsX:Eu has been disclosed wherein the europium dopant is introduced in the CsX by firing CsX with a europium compound containing an halide atom. This phosphor shows high speed and can be used in order to prepare thin binderless screens with good speed.
Nevertheless, since in medical X-ray diagnosis the quest for systems making it possible to lower the patient dose and to keep the image quality still proceeds, there remains a need to have storage phosphors with enhanced speed.
It is a first object of the invention to provide a novel cesium halide phosphor exhibiting high speed.
It is a further object of the invention to provide a method for producing a novel cesium halide phosphor, where such phosphor exhibits a high speed.
It is a still further object of the invention to provide a panel, containing a cesium halide phosphor that exhibits a high speed.
Further objects and advantages of the invention will become clear from the detailed description hereinafter.
The objects of the invention are realized by providing a CsX:Eu phosphor showing upon excitation with light of 370 nm at a wavelength of xcexmax a maximum emission intensity I0 and at xcexmax+30 nm an emission intensity I, such that Ixe2x89xa60.20 I0.
It has been found upon experimentation that storage phosphor screens containing a CsX:Eu phosphor showing a narrow emission spectrum upon UV-excitation did show an enhanced speed when used as storage phosphor screen for radiography. It was found that it was especially important that the emission spectrum of the phosphor upon UV-excitation did have a low emission intensity at the higher wavelength region. Storage phosphor screens or panels incorporating such a phosphor did show a speed increase ranging from 50% to a factor 10 and even 20 when compared to screens incorporating phosphors having upon UV excitation a higher emission intensity at the higher wavelength region.
It has been found upon experimentation that the speed increase of phosphor screens, including a CsX:Eu phosphor with a narrow emission spectrum upon UV-excitation, was very pronounced when said CSX:Eu phosphor, was either a CsBr or CsCl phosphor wherein the Eu doping had proceeded by adding a europium compound containing a halogen atom.
It is clear that this higher speed is very beneficial while it gives more degrees of freedom in finding a trade-off between speed a image quality in computer radiographic system using those screens.
It seems that when a CsX:Eu phosphor with narrow emission spectrum upon UV excitation has absorbed X-ray energy, this energy is, upon stimulation, released as emitted light having also a narrow spectral distribution around a maximum emission, so that more useful light is emitted.
A phosphor showing a narrow emission spectrum upon UV excitation is defined as a phosphor showing upon excitation with light of 370 nm a maximum emission intensity I0 at a wavelength of xcexmax and at xcexmax+30 nm an emission intensity I, such that Ixe2x89xa60.20 I0. It is even more preferred that a phosphor with a narrow emission spectrum upon UV excitation shows upon excitation with light of 370 nm a maximum emission intensity I0 at a wavelength of xcexmax and at xcexmax+30 nm an emission intensity I, such that Ixe2x89xa60.15 I0.
The emission spectrum of a CsX:Eu phosphor upon UV excitation can be narrowed by incorporating in the production process of the phosphor a step of maintaining the phosphor for some time, between 10 minutes to about 15 hours, at a temperature between 80xc2x0 C. and 220xc2x0 C. Further on in this text the step of maintaining the phosphor for a given time at a given temperature will be called xe2x80x9cannealing stepxe2x80x9d, although it is not sure that strictu sensu an xe2x80x9cannealing of the crystal structurexe2x80x9d takes place. Preferably during the xe2x80x9cannealing stepxe2x80x9d, the temperature is kept between 100xc2x0 C. and 180xc2x0 C. and the time between 30 minutes and 10 hours. Most preferably the xe2x80x9cannealing stepxe2x80x9d, is executed at a temperature between 130 and 170xc2x0 C. for between 2 and 5 hours. It was found that to some extent, within the limits given above, time and temperature of the xe2x80x9cannealing stepxe2x80x9d, are interchangeable.
Without being bound to any theory, it is believed that in a phosphor according to this invention the distribution of the europium dopant is changed so that the xe2x80x9cimpurity centrexe2x80x9d is more active.
The xe2x80x9cannealing stepxe2x80x9d can be introduced in the production process of the phosphor at different stages.
Thus the invention encompasses a method for preparing a CsX:Eu phosphor comprising the steps of :
mixing or combining (in another way) CsX with between 10xe2x88x923 mol % and 5 mol % of a europium compound,
heating (e.g., firing) said mixture at a temperature above 450xc2x0 C.;
cooling said mixture to room temperature,
bringing said mixture after cooling to a temperature between 80 and 220xc2x0 C. and
maintaining it at that temperature for between 10 minutes and 15 hours.
In this method after the step of cooling the mixture to room temperature and before the step of bringing said cooled mixture to a temperature between 80 and 220xc2x0 C., a step of grinding to form fine phosphor particles and a step of classifying these phosphor particles can be included.
It is also possible to include the xe2x80x9cannealing stepxe2x80x9d, immediately, after firing, thus the step of cooling the mixture to room temperature is omitted and the mixture is brought to a temperature between 80 and 220xc2x0 C. directly after firing. The invention thus also encompasses a method comprising the steps of
mixing or combining (in another way) CsX with between 10xe2x88x923 mol % and 5 mol % of a europium compound,
heating (e.g., firing) said mixture at a temperature above 450xc2x0 C.;
bringing said mixture after cooling to a temperature between 80 and 220xc2x0 C. and
maintaining it at that temperature for between 10 minutes and 15 hours.
The step of cooling the mixture to room temperature can then be included after the step of maintaining it at that temperature for between 10 minutes and 15 hours.
The xe2x80x9cannealing stepxe2x80x9d, can also be beneficially included when producing binderless phosphor screens comprising a CsX:Eu phosphor.
The method comprises thus a method for producing a binderless CsX:Eu phosphor screen comprising the steps of:
mixing or (otherwise) combining CsX with between 10xe2x88x923 mol % and 5 mol % of a europium compound,
vapour depositing that mixture onto a substrate, forming a binderless phosphor screen,
cooling said phosphor screen to room temperature,
bringing said phosphor screen to a temperature between 80 and 220xc2x0 C. and
maintaining it at that temperature for between 10 minutes and 15 hours.
It is also possible to include the xe2x80x9cannealing stepxe2x80x9d, immediately, after vapour deposition, thus the step of cooling to the phosphor screen to room temperature is omitted and the phosphor screen is brought to a temperature between 80 and 220xc2x0 C. directly after vapour deposition. The invention thus also encompasses a method comprising the steps of:
mixing or (otherwise) combining CsX with between 10xe2x88x923 mol % and 5 mol % of a europium compound,
vapour depositing that mixture onto a substrate, forming a binderless phosphor screen,
bringing said phosphor screen to a temperature between 80 and 220xc2x0 C. and
maintaining it at that temperature for between 10 minutes and 15 hours.
The step of cooling the phosphor screen to room temperature can then be included after the step of maintaining it at that temperature for between 10 minutes and 15 hours.
It is clear that the vapour deposition does not have to start from the precursors, but that it can also proceed by vapour depositing the phosphor itself.